galactic year

{{short description|Unit of time}}

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The galactic year, also known as a cosmic year, is the duration of time required for the Sun to orbit once around the center of the Milky Way Galaxy.[http://www.csi.uottawa.ca:4321/astronomy/index.html#cosmicyear Cosmic Year] {{webarchive|url=https://web.archive.org/web/20140412221436/http://www.csi.uottawa.ca:4321/astronomy/index.html |date=2014-04-12 }}, Fact Guru, University of Ottawa One galactic year is approximately 225 million Earth years.{{cite web|url=http://hypertextbook.com/facts/2002/StacyLeong.shtml|work=The Physics Factbook|last=Leong|first=Stacy|year=2002|title=Period of the Sun's Orbit around the Galaxy (Cosmic Year)}} The Solar System is traveling at an average speed of 230 km/s (828,000 km/h) or 143 mi/s (514,000 mph) within its trajectory around the Galactic Center,http://starchild.gsfc.nasa.gov/docs/StarChild/questions/question18.html NASA – StarChild Question of the Month for February 2000 a speed at which an object could circumnavigate the Earth's equator in 2 minutes and 54 seconds; that speed corresponds to approximately 1/1300 of the speed of light.

The galactic year provides a conveniently usable unit for depicting cosmic and geological time periods together. By contrast, a "billion-year" scale does not allow for useful discrimination between geologic events, and a "million-year" scale requires some rather large numbers.[http://www.vendian.org/mncharity/dir3/geologic_time_galactic Geologic Time Scale – as 18 galactic rotations]